[Edu-sig] this is interesting

[Corey Richardson]

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On 06/08/2011 12:51 AM, michel paul wrote:
>>> def f(n, history = []):
>   history.append(n)
>   return history
>   [...]
> A student wrote me wondering why his function wouldn't 'clear' after being
> called.  He meant to create an empty list and ended up with something like
> this.
> 
> What's a good way to explain what's going on?
> 

It's a very, *very* common newbie mistake. When the function object is
created, the "history=[]" is evaluated and it refers to the same object
during each run of the function. The proper idiom is:

def f(n, history=None):
    if history is None:
        history = []
    # Do stuff

I probably explained that poorly but it's essentially what's going on.
- -- 
Corey Richardson
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